La Niña: Map

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La Niña is a coupled ocean-atmosphere phenomenon
similar to El
Niño. During a period of La Niña, the sea surface temperature across the
equatorial Eastern Central Pacific Ocean will be lower than normal by 0.5 °C. In
the United States, an episode of La Niña is defined as a
period of at least 5 months of La Niña conditions. The name La Niña
originates from Spanish, meaning
"the little girl", analogous to El Niño meaning "the little
boy".

La Niña is the opposite of El Niño, where the latter
corresponds instead to a higher sea surface temperature by
a deviation of at least 0.5 °C. El Niño is famous due
to its potentially catastrophic impact on the weather along both
the Chilean and Australian coasts. Furthermore, La Niña is
often preceded by a strong El Niño.

Effects of La Niña

Regional impacts of La Niña.

La Niña causes mostly the opposite effects of El Niño, for example,
El Niño would cause a dry period in the Midwestern U.S., while La
Niña would typically cause a wet period in this area.

Recent occurrences

There was a strong La Niña episode during 1988-1989. La Niña also
formed in 1995, and in 1999-2000. The last La Niña was a minor one,
and occurred 2000-2001. Currently, there is a moderate La Niña,
which began developing in mid-2007. NOAA confirmed that a moderate
La Niña developed in their November El Niño/Southern Oscillation
Diagnostic Discussion, and that it will likely continue into 2008.
According to NOAA, "Expected La Niña impacts during November –
January include a continuation of above-average precipitation over
Indonesia and below-average precipitation over the central
equatorial Pacific. For the contiguous United States, potential
impacts include above average precipitation in the Northern
Rockies, Northern California, and in southern and eastern regions
of the Pacific Northwest. Below-average precipitation is expected
across the southern tier, particularly in the southwestern and
southeastern states."